A. Field of the Invention
The present invention relates to a method of milking an animal by means of a milking machine, the invention is intended to address problems associated with which are substantially stable during a main milking phase, when the milk flow is substantially constant, and thereafter the milk flow is unintentionally decreased due to a change in teat liner position relative to the animal's teat during a terminating milking phase, during which the milk flow starts decreasing.
B. Description of the Prior Art
The milking machine usually comprises a cluster of teat cups, each teat cup containing a liner forming a pulsation chamber therebetween. During milking the pulsation chamber is subjected to a pulsating pressure varying between a low pressure and a high pressure on the other hand, the interior of the liner is subjected to a constant sub-pressure or vacuum which is substantially equal to said low pressure in the pulsation chamber. When the milking conditions are stable during the main milking phase, the teats are kept in the liners in an intended positions and the milk is yielded from the udder in a substantially constant flow of milk.
During the terminating milking phase, the position of each liner relative to its teat changes unintentionally from the intended position in spite of the fact that the operation of the milking machine is unchanged. This change in position throttles the milk flow passages in the teats, so that the milk flow finally ceases resulting in an undesirable rest amount of milk in the udder.
The cause of said change in position of each liner relative to its teat will be explained below.
When the main part of the milk in the udder has been extracted during milking, the pressure in the udder drops resulting in a decrease of the milk flow from the udder. Under the influence from said constant sub-pressure prevailing in the liner below the tip of each teat and because of said decreasing milk flow, each teat will be slacker and slacker and its friction engagement with the liner will be weaker and weaker. Finally each teat slides deeper into its liner, so that the lips surrounding the upper opening of the latter throttles or constricts the milk flow passage between the interior of the udder and the interior of the teat because of udder tissues sucked into the liner. After a while, the milk flow through the teats is completely stopped by the sucking of udder tissues into the liners, so that a rest amount of milk is left in the udder.
The rest milk cannot be extracted by means of the milking machine without a time-consuming manual manipulation of the teat cups, i.e. the teat cups need to be pulled downwardly. However, before the teat cups can be manually manipulated to gain the rest milk, usually some time will pass during which the animal be dry-milked, i.e. there is no or an insignificant flow of milk in the teats. Such a dry-milking treats the teats ungently and might irritate the animal.
To avoid dry-milking, some known types of milking machines automatically removes the teat cups from the teats as soon as the milk flow has ceased. Thus, the rest milk is not extracted. This means the advantage, besides the advantage of avoiding dry-milking, that no time-consuming manual manipulation of the teat cups is necessary. A drawback is that the lactation production of milk be reduced, the amount of this reduction being dependent on the amount of the rest milk. Another drawback is that leaving the rest milk could increase the risk of infections arising in the udder. As much of the rest milk as possible should always be extracted.
There are attempts made to control a milking machine in order to counteract said unintentional change in the conditions of the milking operation, so that the rest milk is eliminated or at least substantially reduced. According to one such an attempt, during the terminating milking phase, the sub-pressure in the liner is reduced in response to a sensed decrease of the milk flow from the main milk flow to a predetermined part of the latter. By this measure, each liner should remain in a sufficient friction engagement with its teat, since said reduced sub-pressure causes a corresponding reduction of the sub-pressure prevailing in the teat, so that the teat will not be slacked. However, in practice it has been proved that the reduction of the rest milk in most milking occasions is rather insignificant. The benefit is that, because of the reduced sub-pressure, the teats are treated more gently when dry-milking occurs.
The reason for the failure of the above described attempt is that the liner often very rapidly slides upwards on the teat as soon as the milk flow starts decreasing from the main milking phase milk flow. Once the liner has slid up to an upper position on the teat, it remains in that position no matter if the milking machine is controlled, such that the various pressures in the teat cup are changed and/or the pulsation characteristics are changed. Thus, it will often be too late to change the sub-pressure in each liner in response to said change in the milk flow, because each liner has already slid more or less upwards on the teat.